The present invention relates generally to a mask read-only memory (ROM), and, more particularly, to a method to verify the mask for a mask ROM.
Conventionally, the verification for mask ROMs applies ion implantation of binary random code, i.e., xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, to the memory cells of each memory chip and then reads the implanted code out from the memory cells one by one to be further compared with the original implantation pattern so as to determine the chip is perfect or defective. As shown in FIG. 1, for example, a memory chip 10 includes a plurality of memory cells implanted with a xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d code, a plurality of buried diffusion layers 12, and a plurality of polysilicon layers 14, in which the region highlighted with a dotted line indicates a defective memory cell 16. Unfortunately, there can be only one, xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, to be verified in a verification system. For instance, in a system capable of verifying code xe2x80x9c0xe2x80x9d to be defective, a defective cell implanted with code xe2x80x9c1xe2x80x9d may be read a xe2x80x9c0xe2x80x9d out and thus is supposed to be a perfect cell. Particularly, In FIG. 1, the random code implanted into the defective memory cell 16 is xe2x80x9c1xe2x80x9d, and in this case, if the cell implanted with code xe2x80x9c1xe2x80x9d can be verified to be defective, then the defect of the cell 16 can be verified, and thus the mask for the process to manufacture the memory chip 10 can be further determined to be defective. However, in another case that only the cell with implanted code xe2x80x9c0xe2x80x9d, can be verified to be defective, the cell 16 cannot be verified to have a defect, and therefore the mask used to produce the chip 10 cannot be verified defective as well. Since conventional verification method cannot completely verify each memory cell to determine if there is a defect, the photomask used for the process cannot be subsequently accurately verified defective or not.
Accordingly, it is desired a method to completely verify the photomask for mask ROMs.
An object of the present invention is to provide a method that can completely verify the photomask for mask ROMs.
In an embodiment according to the present invention, a verification of the mask for a mask ROM includes implanting a binary random code and its reverse tone into two chips that are manufactured by the same process with the mask, and then testing the two chips and comparing their test results. The two chips used for the verification can be selected from different wafers that are manufactured by the same process with the mask, or from two die regions of a single wafer manufactured by a process with the mask under verified.
For verification of a mask for a mask ROM according to another embodiment, several exclusive random codes are implanted into a plurality of chips, respectively, and the coded chips are then tested for comparison of their test results to determine if the mask is perfect or defective. Likewise, the chips to carry out the verification can be selected from different wafers manufactured by the same process with the mask, or from two die regions of a single wafer with the verified mask.